The so-called "Mamertine Prison", beneath the church of S. Giuseppe dei Falegnami, via di Marforio, Rome, is generally accepted as being identical with "the prison ... in the middle of the city, overlooking the forum", mentioned by Livy (I, xxxiii). It consists of two chambers, one above the other. The lower, known as the Tullianum, was probably built originally as a cistern, whence its name, which is derived from the archaic Latin word tullius, a jet of water  the derivation of Varro from the name of King Servius Tullius is erroneous. The Tullianum is a circular chamber, partly excavated from the rock, and partly built of tufa blocks, each layer of masonry projecting a little over that immediately below so as to form a conical vault. When the upper chamber was constructed, the top of the cone was probably cut off, and the present roof, consisting of a flat arch of tufa blocks, substituted. The upper chamber is an irregular quadrilateral, and contains an inscription recording a restoration made in A.D. 21. Sallust describes the Tullianum, or lower chamber, as a horrible dungeon, "repulsive and terrible on account of neglect, dampness, and smell" (Cat., lv). In the floor of the Tullianum is a well, which, according to the legend, miraculously came into existence while St. Peter was imprisoned here, enabling the Apostle to baptize his jailers, Sts. Processus and Martinianus. The well, however, existed prior to this date, and there is no reliable evidence that the Chief of the Apostles was ever imprisoned in the Tullianum. The Acts of Sts. Processus and Martinianus are of the sixth century. The two chambers are at present connected by a stairway, but originally there was no means of communication between them save a hole in the floor of the upper chamber, through which such famous prisoners as King Jugurtha and the Catiline conspirators were thrown into the lower dungeon, where they died of starvation or were strangled. The name Mamertine Prison is medieval, and is probably derived from the temple of Mars Ultor in the vicinity. The medieval "Itinerary" of Einsiedeln alludes to the "fountain of St. Peter, where also is his prison". From the eighth century the tradition of the Acts of Sts. Processus and Martinianus relative to the imprisonment of St. Peter in the Tullianum was universally accepted; the earliest allusion to the prison in the character of a church is that of Maffeo Veggio, in the fifteenth century, who speaks of it as "S. Petrus in carcere" (St. Peter in prison).

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